Create a function to return number of complex attributes

Details

Description

We have a specific use case to color code WMS based on number of specific attribute in complex features.
e.g. if there is 1 xyz attribute -> color red, if there are 2 xyz attributes -> color blue, etc.

For this I picked up existing LengthFunction in gt-main. I am not really sure the difference between this and strLength function (except strLength is a FilterFunction).

Anyway, I made it return number of attributes for complex attributes. If the text length is what users are after, they should be using strLength.